1. Field of the Invention
This invention relates to a microscope objective lens system, more particularly to a microscope objective lens system qualified as an apochromat class objective lens system having a large numerical aperture (N.A.), high transmittance in the ultra-violet wavelength region, and a low autofluorescence which is suitable for a fluorescent observation.
2. Description of the Related Art
Recently in the biological research field, the fluorescent microscopy has been widely used for observing specific substances in a living cell stained by fluorescent dyes because this way of observation is harmless to the cell.
Generally speaking, the fluorescent microscopy is a manner of observation in which a specimen is irradiated with a light having a short wavelength for excitation and observing the specimen by means of fluorescent light emitted therefrom. A conventional typical wavelength of light for excitation is 365 nm (Fraunhofer i-line) and recently, 340 nm has also become to be used for observing a calcium (Ca) ion in a living body. Other wavelengths included in the ultra-violet and visible regions are also used for excitation.
When the fluorescent microscope is constructed as a vertical illumination type, the objective lens systems exclusively used for the fluorescent microscopy are provided. These kinds of microscope objective lens systems are required to have
(A) a large N.A. on its entrance side (high entrance N.A.),
(B) a large N.A. on its exit side (high exit N.A.),
(C) a favorably corrected longitudinal chromatic aberration,
(D) a favorably corrected image curvature to form a flat image of an object (high PLAN performance),
(E) a high transmittance in the wavelength regions of ultra-violet and visible light, and
(F) a low autofluorescence emittance.
(A) serves to increase the intensity of an illumination light. A bright illumination intensifies the fluorescent light emitted from the specimen. This improves the resolution and allows observing the fine structure of the specimen to a high degree of detail.
(B) serves to increase the brightness of an image formed by the objective lens system. The exit N.A. is calculated by dividing the entrance N.A. by the magnification power of the objective lens system.
(C) serves to allow use of the objective lens system for a wide wavelength range including ultra-violet and visible light because the excitation light and fluorescence have various wavelengths depending on the specimen to be observed.
(D) serves to allow observing a wide field of view at the same time. This improves the ease of operability of the microscope.
(E) serves to increase the intensity of the illumination light and brightness of the image. In the fluorescence microscope, illumination light does not have a broad wavelength band but a specific narrow wavelength band, which is included in the regions of ultra-violet or visible light. Therefore, it is important that the objective lens system has high transmittance in this wavelength region.
(F) serves to obtain a clear image having a high contrast. Autofluorescence of glass material used as a lens element in the objective lens system is a source of undesirable background light that degrades the image quality.
Among the above listed requirements (A) through (F), the features (E) and (F) are typical for an objective lens system used for a fluorescence observation. Therefore, the optical material (optical glass and/or optical crystal) used for this kind of microscope objective lens systems has optical properties to satisfy the requirements (E) and (F), that is, has a high transmittance and a small autofluorescence.
Microscope objective lens systems that meet the requirements (E) and (F) are described in the following publications.
1. Japanese Laid Open Patent application publication Sho 55-79,406
2. Japanese Patent Publication No. 3,140,111 (Japanese Laid Open Patent application publication Hei 5-142,477)
3. Japanese Laid Open Patent application publication Hei 7-230,039
4. Japanese Laid Open Patent application publication Hei 10-274,742
5. Japanese Laid Open Patent application publication Hei 11-174,339
6. Japanese Laid Open Patent application publication Hei 11-352,406
Publication No.1 discloses a fluorescent microscope objective lens system having a magnification power of 20xc3x97 and an N.A. of 0.65. Publication No.2 discloses fluorescent microscope objective lens systems having a magnification power of 100xc3x97 and an N.A of 1.3-1.35. Publication No.3 discloses fluorescent microscope objective lens systems respectively having a magnification power of 100xc3x97 and an N.A. of 1.4, a magnification power of 40xc3x97 and an N.A. of 1.35, and a magnification power of 20xc3x97 and an N.A. of 0.8. Publication No.4 discloses a fluorescent microscope objective lens system having a magnification of 40xc3x97 and an N.A. of 1.3. Publication No.5 discloses fluorescent microscope objective lens systems respectively having a magnification power of 40xc3x97 and an N.A. of 1.3, and a magnification power of 100xc3x97 and an N.A. of 1.3. Publication No.6 discloses a fluorescent microscope objective lens system having a magnification power of 100xc3x97 and an N.A. of 1.3.
These microscope objective lens systems all meet the requirements (E) and (F). However, comparing these to the highest class microscope objective lens system, a so-called plan-apochromat objective in which the numerical aperture is extremely large and the aberrations, particularly the image curvature and longitudinal chromatic aberration including the secondary spectrum are highly corrected, the above described conventional objective lens systems are not satisfactory from the viewpoint of the basic requirements (A) through (D) for a microscope objective lens system.
The objective lens system disclosed in publication No.1 is small in numerical aperture and not satisfactorily corrected in flatness of the image. The objective lens system disclosed in publication No.2 is well corrected in flatness of the image and longitudinal chromatic aberration but the numerical aperture is small. The objective lens system disclosed in publication No.3 is large enough in numerical aperture and well corrected in the longitudinal chromatic aberration but the flatness of the image is not satisfactorily corrected. The objective lens system disclosed in publication No. 4 is not satisfactorily corrected in the secondary spectrum (the Fraunhofer g-line) and the flatness of the image. Among the objective lens systems disclosed in publication No. 5, the one having a magnification power of 100xc3x97 is small in N.A. and unsatisfactorily corrected in the flatness of the image, whereas the other one having a magnification power of 40xc3x97 is unsatisfactorily corrected in the flatness of the image. The objective lens system disclosed in publication No. 6 is small in N.A. and unsatisfactorily corrected in the secondary spectrum (the Fraunhofer g-line).
This invention provides a microscope objective lens system having a large numerical aperture, and well corrected flatness of the image and longitudinal chromatic aberration.
An objective lens system for microscope according to this invention comprises, in order from an object side, a first lens unit including a plurality of cemented lens components and converging light coming from an object, a second lens unit including a negative lens component having a concave surface of strong refractive power on the image side, and a third lens unit including a cemented doublet composed of a negative lens element having a concave surface on the object side and a positive lens element. Further, the microscope objective lens system satisfies the following conditions (1) through (3):
1.65xe2x89xa6ndxe2x89xa61.8xe2x80x83xe2x80x83(1)
xe2x80x8325xe2x89xa6xcexddxe2x89xa641xe2x80x83xe2x80x83(2)
T360xe2x89xa70.5xe2x80x83xe2x80x83(3)
wherein nd and xcexdd are refractive index at the Fraunhofer d-line and Abbe""s number of the positive lens element in the third lens unit, respectively, and T360 is an internal transmittance except a reflection loss at 360 nm of the material of thickness 10 mm forming the positive lens element in the third lens unit.
The objective lens system according to this invention is particularly suitable for fluorescence microscopy.
Other features and advantages of this invention will become apparent from the following detailed description of the examples when taken in conjunction with the accompanied drawings and appended claims.